The present invention relates to a burn through resistant nonwoven mat, and in particular to a burn through resistant nonwoven mat that adds significant burn through resistance to the thermal and acoustical insulation blanket systems used in commercial aircraft and in other applications requiring burn through properties of the type or similar to those properties currently required for commercial aircraft.
U.S. Pat. No. 5,759,659, issued Jun. 2, 1998, (hereinafter xe2x80x9cthe ""659 patentxe2x80x9d) describes insulation blankets with added burn through resistance. The ""659 patent seems to describe a blanket comprising an insulation composite encased in a heat-sealable polyolefin film with a high temperature-resistant layer adjacent to the insulation. The ""659 patent suggests that it may be appropriate for use in an aircraft as an insulation blanket. However, the ""659 patent does not disclose an insulation blanket xe2x80x9csystemxe2x80x9d that meets the combined thermal, acoustical, component and composite small scale flammability, fire barrier, fire propagation, smoke toxicity, moisture management, weight, fabricate-ability, health and cost requirements that have been established by commercial aircraft manufactures and aircraft regulatory agencies in the United States. Examples of why the invention described by the ""659 patent does not meet the combined required properties for use in commercial aircraft follow.
The ""659 patent cites the BSS 7323 Cargo Liner Burn Test to demonstrate the xe2x80x9cgoodxe2x80x9d burn through resistance of the invention described in the ""659 patent. However, the ""659 patent""s xe2x80x9cgoodxe2x80x9d composite examples may not meet the new Federal Aviation Administration (FAA) medium scale burn through test (FAA test as presently defined by www.Fire.tc.faa.gov and hereinafter xe2x80x9cthe FAA medium scale burn through testxe2x80x9d). The FAA medium scale burn through test subjects a hot side major surface of the composite sample being tested to the flame of an oil burner that generates a temperature of approximately 1100xc2x0 C. The discharge end of the oil burner nozzle is positioned 10 cm from and directed toward the hot side major surface of the composite sample, and generates a pulsating high pressure flame front. The FAA medium scale burn through test measures cold side heat flux at the cold side of the composite sample. To pass the FAA medium scale burn through test an insulation system must prevent both visible burn through and a cold side heat flux exceeding 1.5 btu/ft2 for four minutes.
For aircraft applications, the ""659 patent uses the Federal Aviation Regulations (FAR) part 25.853A or the Boeing Material Technology Test BSS 7230, set up on a laboratory scale, vertical burn test to determine the small scale flammability characteristics of components. The ""659 patent states xe2x80x9cA sample was said to have passed the vertical burn test if the sample did not burn along its entire length during the 12 second test periodxe2x80x9d. Contrary to this statement, none of the requirements established by Airbus or Boeing permit more than two thirds of the sample length to burn. BMS 8-48V for fiberglass insulation requires 10 seconds maximum burn time, one third maximum burn length and no drips. BMS 8-142V for composites of insulation and covering film requires 2 seconds maximum burn time, two thirds maximum burn length, and 5 seconds to extinguish drips, respectively. Also, none of the examples or preferred variations in the ""659 patent meet the two thirds maximum burn length requirement when tested as an actual or simulated composite fuselage insulation part as required by FAR.
Thus, there remains a need for an aircraft blanket xe2x80x9csystemxe2x80x9d that responds to and meets all of the regulatory, aircraft manufacturer and aircraft operator requirements and expectations. The invention set forth in this patent application is such a system.
One composite that has been evaluated and has passed the FAA medium scale burn through test uses one or more layers of a nonwoven mat, made of fibers commercially available from The 3M Company of St. Paul, Minn. under the trade designation xe2x80x9cNEXTELxe2x80x9d, as a fire barrier in the insulation blanket. This NEXTEL fiber mat is positioned between the hot-side film and the first layer of MICROLITE AA fiber glass blanket that is commercially available from Johns Manville International, Inc. of Denver, Colo. This composite performs well when compared to the insulation blanket systems discussed above because this mat is made from nonwoven ceramic fibers that are bonded together with a high temperature ceramic binder. The addition of the high temperature ceramic binder helps the mat retain its form and strength for at least four minutes during exposure to the 1100xc2x0 C. temperature and pulsating high pressure flame front in the FAA medium scale burn through test.
Although this NEXTEL mat/MICROLITE AA fiber glass blanket composite passes the FAA medium scale burn through test, there are four properties of the NEXTEL fiber mat used in the composite that make the composite unsuitable as a xe2x80x9csystemxe2x80x9d solution and are likely to limit wide scale commercial acceptance of such a composite. First, the NEXTEL fiber mat is not xe2x80x9cuser friendlyxe2x80x9d because NEXTEL fibers have a tendency to pull moisture and oil from the hands of the people fabricating insulation composites. Second, the NEXTEL fiber mat is fairly rigid and can crack upon bending as well as separate at perforations caused while sewing the mat; and the NEXTEL fiber mat does not possess the xe2x80x9cdrapexe2x80x9d characteristics of the insulation blanket components currently used in thermal and acoustical insulation blanket systems for aircraft thereby making it difficult to fabricate an insulation composite with the NEXTEL fiber mat and install such a composite within an aircraft fuselage. The NEXTEL fiber mat""s third undesirable property is the lack of moisture resistance required by Boeing Material Technology test BMS 8-48. The final problem with the NEXTEL fiber mat relates to the cost of the NEXTEL fiber mat. The cost of the primary component of this 50-70 g/m2 mat (the NEXTEL fiber) is about $176/kg. Thus, the cost for the fibers alone in the mat is at least $8/m2.
The preferred embodiments of the present invention address all of the unwanted properties of the NEXTEL fiber mat used in the NEXTEL fiber mat/MICROLITE AA glass fiber blanket insulation composite and retain the excellent burn through resistance required to pass the FAA medium scale burn through test. The preferred embodiments of the present invention also address all of the other xe2x80x9csystemxe2x80x9d requirements and expectations.
The burn through resistant nonwoven mats of the present invention (for use in thermal and/or acoustical insulation blanket systems, such as but not limited to the thermal and/or acoustical blanket insulation systems of aircraft fuselages) are preferably made up of non-respirable and/or biosoluble base fibers; are capable of retaining their integrity and dimensional stability during four minutes of exposure to a fluctuating high pressure flame front at a temperature of 1100xc2x0 C.; and are capable of passing the FAA medium scale burn through test. Examples of non-respirable base fibers which make up the nonwoven mat are quartz fibers, aluminosilicate ceramic oxide fibers, aluminoborosilicate ceramic oxide fibers, alumina ceramic oxide fibers, partially oxidized pitch based fibers, and partially oxidized polyacrylonitrile fibers having mean diameters greater than 6 microns. Examples of biosoluble base fibers (fibers that are not durable in physiological fluids) are biosoluble glass fibers.
Preferably, the burn through resistant nonwoven mats also include a lubricant sizing with a water repellent additive. The randomly oriented entangled base fibers forming the burn through resistant nonwoven mat, through the physical characteristics of the base fibers and their entanglement, normally provide the nonwoven mat with the required integrity. However, for applications where additional mat integrity is required e.g. for processing and/or handling, a high temperature resistant binder can be used to bond the randomly oriented entangled base fibers together to increase the integrity of the nonwoven mat; secondary microfibers that are entangled with each other and the base fibers of the nonwoven mat can be included in the nonwoven mat to increase the integrity of the mat; or both a high temperature resistant binder and the secondary microfibers can be used to increase the integrity of the nonwoven mat.